1. Field of the Invention
The present invention relates to a photoconductive device and a method for operating the same and, in particular, to a photoconductive device mainly composed of amorphous semiconductors and including a photoconductive layer having significantly raised sensitivity and blocking contact under the state that fine photo response is maintained, and to its operating method.
Photoconductive devices according to the present invention include solid-state photoconductive devices of laminated photoconductive layer type such as photocells, one dimensional image sensors and two dimensional image sensors, and photoconductive devices represented by photoconductive image pick-up tubes. Further, photoconductive devices according to the present invention include photoconductive devices used to read out the signal charge by means of electronic switches or the like and photoconductive devices used for optical communication or the like.
2. Description of the Related Art
Photoconductive devices composed mainly of amorphous semiconductors include solid-state photoconductive devices of laminated photoconductive layer type such as photocells, one dimensional image sensors described in JP-A-52-144992, laid-open on Dec. 2, 1977, for example, and two dimensional image sensors composed of combination of solid-state drive circuits and amorphous photoconductors disclosed, for example, JP-A-49-24619, laid-open on Mar. 5, 1974 (corresponding to Japanese Patent Application No. 47-59514, filed Jul. 3, 1972). Such photoconductive devices also include photoconductive image pick-up tubes. In solid-state photoconductive devices of laminated photoconductive layer type such as photocells and one dimensional image sensors among the prior art devices, an electrode having such contact as to block the charge injection is usually used with respect to the photoconductive layer in order to attain fine photo response. However, it has heretofore been impossible to realize a device which is capable of extracting the signal charge exceeding the number of carriers generated by the incident light. That is to say, the gain of photoelectric conversion was below unity.
As targets for photoconductive image pick-up tubes, so-called targets of blocking type described in JP-A-49-24619, for example, and so-called targets of injection type are used. The target of blocking type has such a structure that charge injection from the signal electrode side and the electron beam scanning side is prevented. The target of injection type has such a structure that the charge is injected from the signal electrode side and/or the electron beam side. The target of blocking type has a feature that the lag can be reduced. Because of absence of multiplying function at the photoconductive layer, however, a highly sensitive target of blocking type having a gain larger than unity has not heretofore been obtained.
On the other hand, more electrons than incident electrons can be introduced into an external circuit in accordance with the principle of the target of injection type. Accordingly, there is a possibility of increasing the sensitivity so as to attain a gain larger than unity. A highly sensitive image pick-up tube using a monocrystalline semiconductor target plate of np structure has already been proposed in JP-A-43-18643 (published on Aug. 13, 1967). There has also been proposed a highly sensitivre image pick-up tube having an electron injection and recombination layer at the beam scan side of the photoconductive layer in order to inject scanning electrons and recombine scanning electrons with holes (JP-A-62-2435, laid-open on Jan. 8, 1987 corresponding to Japanese Patent Application No. 60-140288, filed on Jun. 28, 1985).
In accordance with any of the above-described techniques having a high sensitivity of a target of an image pick-up tube of photoconductive type to attain the gain larger than unity, however, a part of scanning electrons is injected into the target of the image pick-up tube. In principle, therefore, the effective storage capacitance of the target is disadvantageously increased and hence the lag is increased.
The image pick-up tube having a semiconductor target plate described in the aforementioned JP-A-43-18643 must satisfy the condition T.sub.t &lt;T.sub.n .ltoreq.T.sub.e, where T.sub.t represents the average scanning time required for scanning electrons which have reached a p-type monocrystalline semiconductor layer to reach a signal electrode through an n-type monocrystalline semiconductor layer, and T.sub.n and T.sub.e represent the average life of electrons in the p-type monocrystalline semiconductor layer and scanning time required for the scanning electron beam to scan one picture element, respectively. In addition, it is difficult to obtain a monocrystalline semiconductor substrate of good qualtiy. In case Si single crystal is used as the monocrystalline substrate, the resistivity of the substrate is low and hence the np structure must be separated in the mosaic form as described in the above described JP-A-43-18643. It was not desirable in raising the resolution of the image pick-up tube.